1. Field of the Invention
This invention relates to a controlling apparatus for controlling the power to be supplied to a load from a high frequency high voltage power source used suitably for a corona discharge processing apparatus or a like apparatus.
2. Description of the Related Art
The applicant of the present invention has proposed, in Japanese Patent Laid-Open Application No. Heisei 1-218356, a method of adjusting the high frequency output power of a high voltage transformer of a high frequency high voltage power source, wherein the power from a commercial ac power supply is rectified by a rectifier circuit and the dc power obtained from the rectifier circuit is switched by means of an invertor and boosted by the high voltage transformer, by varying the switching excitation frequency supplied to a switching semiconductor element connected between the dc power source and the high voltage transformer.
When a high voltage from the high frequency high voltage power source is applied to a discharge electrode to perform surface treatment of a plastic film or a like material by corona discharge from the discharge electrode, the high voltage transformer, the discharge electrode and the high voltage wiring line cooperatively form a dc resonance circuit and make a resonance load as viewed from the invertor of the high frequency high voltage power source. Therefore, where a popular semiconductor element is employed as the switching semiconductor element between the dc power source and the high voltage transformer, if it does not perform its switching operation at or around the zero resonance current while it is always held in synchronism with the resonance current, then heat generation or a breakdown of the element is caused by the switching loss of the semiconductor element. However, the method described above only discloses a general method of merely varying the switching excitation frequency, but does not pay particular attention to the problem. Further, the method does not provide any particular adjustment of the output voltage and cannot realize continuous adjustment over a wide range.